1. Field of the Invention
This invention relates to and has among its objects the provision of new methods for removal of superoxy-anion-forming metals from materials containing the same. Further objects of the invention will be evident from the following description wherein parts and percentages are by weight unless otherwise specified. The abbreviation "ppm" means parts per million and "ppb" means parts per billion.
2. Description of the Prior Art
Chromium and other metal compounds are widely used in American industry. For example, large amounts of chromium-containing materials are employed in the steel industry, in metal finishing and electroplating operations, in dyeing of fabrics, in hide and leather tanning, etc.
The waste materials which arise from the use of metal compounds must be disposed of, and this presents a problem from an ecological standpoint. Many of these compounds are extremely toxic, thus a safe concentration and distribution of these compounds in the environment must be maintained. Furthermore, metal compounds, particularly chromium compounds, impair biodegradation of organic matter in the biological processes of sewage treatment. Finally, the metals tend to accumulate and be concentrated in the food chain, posing a threat to the well-being of humans and other animals.
To control the above detrimental results from the use of metal compounds, federal, state, and municipal regulations have been established for allowable concentrations of these metals in waste streams which can be released into the environment, i.e., into waterways such as streams, rivers, lakes, and the like. Such regulations are expected to be made even more stringent in the future. It is desirable to reduce the chromium level, e.g., in the waste to less than 50 ppb (the present standard for drinking water).
Present methods employed to lower the chromium content of waste streams involve conversion of hexavalent chromium ion (the most prevalent form of chromium in waste streams) to the trivalent form followed by precipitation as chromium hydroxide. The precipitate is then removed by filtration or as settled sludge. This method requires wasteful use of reagents such as sulfur dioxide as the reducing agent and alkali as the precipitating agent. In addition, the reduction and precipitation steps must be adequately monitored. Lastly, both the filtration, and settling steps are cumbersome and require inordinate amounts of space.